1. Field of the Description
This application relates generally to the field of power systems, and more particularly to high reliability power systems with redundant power supply units and/or the capability of operating from more than one independent electrical power source.
2. Relevant Background
Power systems for data processing and communications equipment are expected to provide power to electronic equipment with extremely high reliability. For example, data center power systems may be expected to provide greater than 99.9%, 99.99%, or even 99.999% availability of power for the electronic equipment of the data center. To provide this high reliability, these systems typically implement fault-tolerant architectures with redundant power paths through redundant power supply units and/or the capability of operating from more than one independent electrical power source.
Redundancy in power supply units may be provided by including additional power supply units. For example, power systems typically implement N+1 redundancy, meaning that if N power supply units are required to supply the required output load, the power system includes an additional power supply unit besides the required N power supply units. In this way, if any one of the power supply units is out of operation because of component failure or system maintenance, the power system can still supply the rated load.
Power supply unit redundancy can only improve the reliability of the system up to the reliability of the single input power source. Therefore, many power systems employ redundancy of input power sources. These systems may have multiple independent primary power sources or a primary power source and a backup power source such as a generator and/or battery back-up.
Some fault tolerant power systems use multiple independent alternating current (AC) input power sources. These systems are expected to operate properly from both AC input power sources or from either independent AC input source if the other source fails or is out of tolerance. In some systems, the output load should be shared between both independent AC input sources when both sources are present.
Power systems that accept multiple independent power sources must maintain isolation between each power source. One reason for power source isolation is that independent AC and/or direct current (DC) power sources may have differences in the reference point, voltage, frequency, and/or phase from one another. In addition, multiple input power sources must be isolated so that excessive current does not flow between the independent power sources. For example, a power supply accepting multiple independent AC inputs may be required to accept AC voltages from the input sources up to 240 volts and deliver tens of amps of current to a load without allowing more than a few thousandths of an amp of current to flow between the input sources.
High isolation between independent input sources is also a safety requirement. Non-isolated inputs may allow voltage to feed through from one input source to another input source, which may present an unacceptable safety hazard. For example, when an AC power cord is unplugged from a wall outlet, the prongs of the plug are exposed and easily touched. Therefore, voltage from other input power sources should not feed through to the exposed prongs of the unplugged AC power cord.
One power system architecture that provides multiple isolated input sources uses separate single-input power supply units rated for the full load providing power from each input source. This type of redundancy may be called N+N redundancy because, if N power supply units are required to supply the load from one input power source, N+N power supply units would be required to provide the load from two independent input power sources.
Another approach is to use dual-input power supply units with isolated input power paths. One type of dual-input power supply converts multiple AC input voltages from independent inputs to an isolated secondary DC voltage, which may then be combined. This type of dual-input power supply provides high isolation between inputs without degrading efficiency. However, this type of power supply does not have a significant advantage over the N+N redundant power system architecture with regard to cost or system volume.
A second type of dual-input power supply uses transformer isolation in each input power path within or just following a power factor correction stage of the power path. This approach avoids duplication of the low voltage output conversion circuits but reduces efficiency in each power path by about five percent by including two transformers in each input power path. Additionally, each power supply requires three transformers, all rated to the full output load of the power supply.
A third type of dual-input power supply uses relay switching between the input sources. Relay switching provides high isolation between input sources but has other disadvantages including difficulty in achieving a clean transfer under all possible fault conditions for AC input sources.
Accordingly, existing approaches to providing redundant input power sources to power systems involve a large increase in system cost or have drawbacks in switching between input power sources that may limit applicability in high-reliability power systems.